1. Field
This disclosure relates to ortho-mode transducers for coupling orthogonally polarized TE11 modes into or from coaxial waveguides.
2. Description of the Related Art
Satellite broadcasting and communications systems commonly use separate frequency bands for the uplink to and downlink from satellites. Additionally, one or both of the uplink and downlink typically transmit orthogonal right-hand and left-hand circularly polarized signals within the respective frequency band.
Typical antennas for transmitting and receiving signals from satellites consist of a parabolic dish reflector and a coaxial feed where the high frequency band signals travel through a central circular waveguide and the low frequency band signals travel through an annular waveguide coaxial with the high-band waveguide. Note that the terms “circular” and “annular” refer to the cross-sectional shape of each waveguide. An ortho-mode transducer may be used to launch or extract orthogonal TE11 linear polarized modes into the high-band and low-band coaxial waveguides. A linear polarization to circular polarization converter is commonly disposed within each of the high-band and low-band coaxial waveguides to convert the orthogonal TE11 modes into left-hand and right-hand circular polarized modes for communication with the satellite.
An ortho-mode transducer (OMT) is a three-port waveguide device having a common waveguide coupled to two branching waveguides. Within this description, the term “port” refers generally to an interface between devices or between a device and free space. A port may include an interfacial surface, an aperture in the interfacial surface to allow microwave radiation to enter or exit a device, and provisions to mount or attach an adjacent device.
The common waveguide of an OMT typically supports two orthogonal linearly polarized modes. Within this document, the terms “support” and “supporting” mean that a waveguide will allow propagation of a mode with little or no loss. In a feed system for a satellite antenna, the common waveguide may be a circular waveguide or an annular waveguide. The two orthogonal linearly polarized modes may be TE11 modes which have an electric field component orthogonal to the axis of the common waveguide. Two precisely orthogonal TE11 modes do not interact or cross-couple, and can therefore be used to communicate different information.
The common waveguide terminates at a common port aperture. The common port aperture is defined by the intersection of the common waveguide and an exterior surface of the OMT.
Each of the two branching waveguides of an OMT typically supports only a single linearly polarized mode. The mode supported by the first branching waveguide is orthogonal to the mode supported by the second branching waveguide. Within this document, the term “orthogonal” will be used to describe the polarization direction of modes, and “normal” will be used to describe geometrically perpendicular structures.
The two branching ports and the associated waveguides are commonly termed the “vertical” and “horizontal” ports. The terms “horizontal” and “vertical” will be used in this document to denote the two orthogonal modes and the waveguides and ports supporting those modes. Note, however, that these terms do not connote any particular orientation of the modes or waveguides with respect to the actual physical horizontal and vertical directions.
In order to minimize coupling between orthogonal TE11 modes, the OMT that launches the TE11 modes must provide high isolation between the orthogonal TE11 modes, and must avoid launching or coupling the TEM (transverse electro-magnetic) mode and higher order modes.
Throughout this description, elements appearing in views of the OMT are assigned three-digit reference designators, where the most significant digit is the figure number where the element was first introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.